The invention concerns a corrector for chromatic and aperture aberration correction in an electron microscope with six multipoles which are disposed in the optical path, one after the other, and symmetrically with respect to a symmetry plane in such a fashion that the first three are disposed upstream of the symmetry plane and the second three are disposed downstream of the symmetry plane, and all of which are used for generating quadrupole fields and octupole fields, wherein the quadrupole fields of all six multipoles are consecutively rotated through 90° with respect to one another and are point-symmetrical with respect to the point of intersection between the optical axis and the symmetry plane, wherein an exchange symmetry of the axial fundamental rays with the symmetry plane as mirror plane for the exchange is generated, and wherein both the axial and the off-axial fundamental rays are reunited at the end of the corrector, and chromatic aberration correction can be achieved through cooperation of the quadrupole fields, designed in the form of magnetic and electrical fields, of the third and fourth multipoles, and aperture aberration correction and off-axial aberrations, produced by the corrector itself, are corrected through adjustment of octupole fields which exert the same direction of force on the electrons in the principal sections as the quadrupole fields.
The purpose of a corrector of this type is that the chromatic and aperture aberrations of the optical components of the microscope can be compensated for. Compensation using such correctors not only compensates for aberrations of the beam source and the lenses of the electron microscope but also for aberrations caused by the corrector itself.
The basis of the function of all correctors in particle optics is the discovery by O. Scherzer (O. Scherzer: “Sphärische and chromatische Korrektur von Elektronen-Linsen” (spherical and chromatic correction of electron lenses) OPTIK, DE, JENA, 1947, pages 114-132, XP002090897, ISSN: 0863-0259) that chromatic and aperture aberrations can be corrected for particle beams by generating non-rotationally symmetrical fields using quadrupoles for forming a non-circular beam, which is then aberration-corrected and made round again by oppositely acting fields. The non-circular beam areas may be astigmatic intermediate images or an elliptical beam area. These are then used for aberration correction using multipole fields, such as hexapole fields, octupole fields or twelve-pole fields using multipoles. These fields are then exclusively used for performing corrections and have no influence on the fundamental ray behavior. Only chromatic aberration correction is performed with quadrupole fields, wherein one electric and one magnetic quadrupole cooperate in each of both the x and the y directions. O. Scherzer sets the conditions under which such aberration corrections can be achieved (loc.cit.). These conditions, which are called Scherzer theorem, form the basis of any chromatic and aperture aberration correction in particle optics.
DE 10 2007 049 816 B3 discloses a corrector which consists of five multipole elements but which itself causes a considerable amount of aberrations of higher order, which aggravates aberration compensation.
DE 42 04 512 A1 proposes a corrector of the above-mentioned type which causes less aberrations of higher order but does not eliminate azimuthal coma, which is a requirement for a corrector to be used in a transmission electron microscope. This corrector is moreover extremely sensitive to field fluctuations of the quadrupole fields. These field fluctuations, which are also called “noise”, are caused by fluctuations in the current or voltage supply. The sensitivity to such fluctuations turned out to be sufficiently large in the test phase of a corrector constructed in accordance with this document, that it was impossible to handle this sensitivity with reasonable technical expense by means of highly stable supply of electric energy. For this reason, both the attempt of realizing this corrector as well as the subject patent application were abandoned.
It is therefore the underlying purpose of the invention to further develop a corrector of the above-mentioned type in such a fashion that it is suited for transmission electron microscopes and is insensitive to fluctuations in the electrical energy supply.